Radiolysis and Photolysis of Icy Satellite Surfaces: Experiments and Theory

The transport and exchange of material between bodies in the outer solar system is often facilitated by their exposure to ionizing radiation. With this in mind we review the effects of energetic ions, electrons and UV photons on materials present in the outer solar system. We consider radiolysis, photolysis, and sputtering of low temperature solids. Radiolysis and photolysis are the chemistry that follows the bond breaking and ionization produced by incident radiation, producing, e.g., O-2 and H-2 from irradiated H2O ice. Sputtering is the ejection of molecules by incident radiation. Both processes are particularly effective on ices in the outer solar system. Materials reviewed include H2O ice, sulfur-containing compounds (such as SO2 and S-8), carbon-containing compounds (such as CH4), nitrogen-containing compounds (such as NH3 and N-2), and mixtures of those compounds. We also review the effects of ionizing radiation on a mixture of N-2 and CH4 gases, as appropriate to Titan's upper atmosphere, where radiolysis and photolysis produce complex organic compounds (tholins).